Natural emissions of gaseous elemental mercury, GEM, are estimated to be as large as or larger than the total anthropogenic mercury emissions to the atmosphere. Measurements of dissolved gaseous elemental mercury concentrations in surface waters are higher than predicted by Henry's constant and the atmospheric concentrations. Air quality models currently parameterize the atmosphere-surface water exchange of mercury by eliminating the atmospheric deposition and parameterizing the evasion as a function of meteorological parameters and/or constant surface water dissolved GEM concentration. Recent flux chamber measurements indicate a seasonal pattern in the atmospheric – surface water exchange of mercury correlating with incoming solar radiation during the summer and with wind intensity during the winter. A mass conservative physically descriptive multimedia bi-directional mercury exchange model was developed using the U.S. Environmental Protection Agency's Community Multiscale Air Quality (CMAQ) modeling system to capture these processes. The wind driven atmosphere – surface water mercury fluxes are being parameterized using a two – film turbulent diffusion model and a surface water photo-reduction scheme is used to better capture the enrichment of surface water dissolved GEM concentrations. Both of these processes require mercury concentrations in the surface waters and a dynamic surface media layer to simulate surface accumulation and depletion of mercury from deposition and evasion respectively has been added to CMAQ. The development of the model and preliminary modeling results will be presented.